This invention relates to a chemical method for removing certain difficult to remove olefinic impurities from epichlorohydrin. More particularly, this invention is directed to a catalytic hydrogenation process for selectively removing beta-chloroacroleins and 5,6-epoxyhexene-1 from a high purity epichlorohydrin stream e.g., greater than about 90% by weight epichlorohydrin, obtained by epoxidation of allyl chloride with an organic hydroperoxide in the presence of a catalyst of an inorganic oxygen compound of silicon in chemical combination with an oxide or hydroxide of titanium.
Epichlorohydrin is a material of commerce having particular utility as a starting material for the manufacture of epoxy resins. In the past, commercial routes to epichlorohydrin have been essentially based on the chlorohydrination of allyl chloride (addition of hypochlorous acid to allyl chloride to form a glycerol dichlorohydrin intermediate followed by alkali metal promoted dehydrohalogenation of the intermediate to epichlorohydrin). More recently, environmental constraints coupled with the energy situation have turned the industry's attention to alternative synthetic routes to epichlorohydrin which are more attractive from effluent handling and/or energy consumption standpoints. One alternate process having considerable appeal affords epichlorohydrin directly via the epoxidation of allyl chloride with an organic hydroperoxide in the presence of a heterogeneous, titanium based catalyst. In this process, which is broadly described in U.S. Pat. No. 4,021,454, a molar excess of allyl chloride is reacted with a hydrocarbon hydroperoxide such as ethylbenzene hydroperoxide or tertiary butyl hydroperoxide in the presence of a catalyst comprising an inorganic oxygen compound of silicon in chemical combination with an oxide or hydroxide of titanium to afford epichlorohydrin in high yield and selectivity with concomitant conversion of hydroperoxide to the corresponding alcohol. In a typical operation, the epichlorohydrin is recovered from the epoxidation reaction effluent by a series of distillations in which the excess allyl chloride and lighter impurities are initially removed as an overhead stream with the bottoms being passed to a second distillation where crude epichlorohydrin is separated from the alcohol product followed by one or more distillations to upgrade the crude epichlorohydrin e.g. by sequential removal of light and heavy end impurities, to give a purified material having a purity of greater than 95% by weight epichlorohydrin. After suitable upgrading, the excess allyl chloride recovered from the reaction effluent is recycled back to the epoxidation reaction zone and if desired, the alcohol can be recovered as a coproduct of the process or reconverted to hydroperoxide via procedures such as dehydration to olefin, hydrogenation of the olefin and oxidation to hydroperoxide.
While the aforementioned process employing a heterogeneous titanium containing epoxidation catalyst is quite attractive in terms of reduced energy consumption and the production of a smaller volume, more treatable aqueous effluent than the conventional chlorohydrination process, it is not devoid of problems. One problem area is the apparent unavoidable production of certain olefinic byproducts in the epoxidation and/or subsequent processing steps which closely resemble epichlorohydrin in physical properties and therefore are difficult, if not impossible, to remove by conventional physical separation techniques. These process byproducts, specifically cis- and trans-beta-chloroacroleins and 5,6-epoxyhexene-1, are present only in minor amounts in the recovered epichlorohydrin i.e., the combined impurity concentration typically being less than 2% by weight; however, stringent purity specifications for the finished epichlorohydrin coupled with certain undesirable traits of the impurities themselves, such as color and reactivity in end use applications of the finished epichlorohydrin, make efficient removal of these impurities almost essential.
From the foregoing, it is apparent that considerable advantage would be obtained if an economic and efficient technique could be developed for substantially complete removal of the beta-chloroacrolein and 5,6-epoxyhexene-1 impurities from the recovered epichlorohydrin. Further, it would be even more desirable if this removal of impurities could be effected with minimal destruction of the recovered epichlorohydrin.